It is normal for the rotor of a superconducting synchronous machine to be located inside a cryogenic chamber (often called a cryostat) so that the superconducting material that is used in the rotor windings can be maintained below its critical superconducting temperature. For a high temperature superconducting (HTS) material such as BSCCO-2223 (Bi(2-x)PbxSr2Ca2Cu2O10) or YBCO (YBa2Cu3O7-δ) the temperature in the cryostat can be anywhere between 27 and 110 K. The rest of the superconducting synchronous machine will remain at an ambient temperature of about 300 K. For the purposes of this patent specification, the term “cold environment” will be used to refer to the low temperature environment inside the cryostat and the term “warm environment” will be used to refer to the ambient temperature environment outside the cryostat.
It is essential for the operation of the superconducting synchronous machine that the rotor windings are excited by supplying them with a field current. In a typical superconducting synchronous machine the full field current is supplied to the rotor windings through a pair of transfer leads that pass from the warm environment to the cold environment through a wall of the cryostat. The field current is provided by a power supply and can be supplied to the transfer leads using a pair of slip rings and brush contacts. The transfer leads are designed to minimise the stray heat transfer between the warm environment and the cold environment to reduce any possible adverse impact on the performance requirements of the cryogenic cooling system. However, the transfer leads must also have a significant cross sectional area if they are to carry the full field current, which may be between ten and two thousand amperes. Increasing the cross sectional area also increases the amount of stray heat transfer through the transfer leads. Therefore, in practice, the design of the transfer leads must be a compromise between the need to maximise the current carrying capacity whilst at the same time trying to minimise stray heat transfer.
U.S. Pat. No. 6,420,842 describes an exciter assembly for supplying a field current to the rotor windings of a superconducting synchronous machine. The exciter assembly includes a transformer 106 having a primary winding 108 and a secondary winding 112. The primary winding 108 receives current from an ac power source 110 that is preferably a high frequency excitation source (e.g., 400 Hz to 2 kHz). The transformer 106 is therefore fed by a switched mode power supply. In practice, it will be readily appreciated that the transformer 106 may or may not be a pulse transformer depending on whether or not the switched mode power supply is filtered.
An ac voltage is supplied from the secondary winding 112 to a full wave bridge rectifier 114 whose dc output is supplied to storage capacitor 116. The dc voltage across the storage capacitor is not described as being regulated in any particular way. The dc voltage is converted to a switched mode regulated current that flows in field winding 102 by rotating semiconducting power devices 120, 122 and 138 which can either be located in a cryogenic environment or a warm environment as required. When the rotating semiconductor power devices 120, 122 and 138 are located in a cryogenic environment, a pulsed current flows in the transfer leads that pass between warm and cryogenic environments. When the rotating semiconductor power devices 120, 122 and 138 are located in a warm environment, a substantially non-pulsing current flows in the transfer leads that pass between warm and cryogenic environments. In both cases, the switched mode regulator power semiconductor devices are in a rotating environment and carry field current. A field current regulation process using a telemetry link comprising stationary machine controller interface 134 and a rotating field coil controller interface 130 that employ pulse code modulated carrier infrared optical transmission and reception in order to bi-directionally transfer regulator signals between stationary and rotating environments. A current sensor 132 is located in the rotating environment and it is necessary to transfer data from this sensor via the telemetry link to enable closed loop regulation of field current to be performed.
Accordingly, there is a need for an alternative exciter assembly that does not require switched mode regulator power semiconductor devices and a field current transducer to be in a rotating environment and for these to have to communicate with the stationary environment in order to enable closed loop regulation of field current to be performed.